1. <Field of the Invention>
The present invention relates to a vehicle headlamp. More particularly, the present invention relates to a projector type vehicle headlamp having a variable light distributing function capable of changing a light distribution of the headlamp according to a running state of a vehicle.
2. <Related Art>
There is a conventional vehicle headlamp which has a projector type lighting device unit composed in such a manner that light (light source light) emitted from a light source bulb arranged on an optical axis extending in the longitudinal direction of a vehicle is reflected to the front side by a reflector while the light is being converged toward the optical axis and the thus reflected light is irradiated onto the front side of a lighting device through a projection lens provided in the front of the reflector.
This projector type lighting device unit is usually composed as follows. Between the projection lens and the reflector, a shade is arranged which is capable of shading a part of the reflected light sent from the reflector so that an unnecessary part of the reflected light can be shaded by the shade according to a light distribution characteristic such as a low beam light distribution pattern (a passing beam light distribution pattern), that is, it is common that a cut-off line is formed in an upper end portion of a desired light distribution pattern in this way.
In this case, the shade is stationary. Therefore, for example, when the shade is set for the low beam light distribution pattern, this lighting device unit can be only exclusively used for the low beam. It is impossible to change over the lighting device unit to the use for an upper beam (a driving beam).
In order to solve the above problems, a headlight for vehicle use is proposed which is capable of obtaining the most appropriate light distribution characteristic for both the low beam and the upper beam as follows. The shade is a movable shade capable of moving between a low beam position and an upper beam position. In the case of the low beam position, an upper end edge of the shade is positioned at a focus of the projection lens. In the case of the upper beam position, the upper end edge of the shade is positioned at a position which is appropriately displaced from the focus of the projection lens. Due to the foregoing, it is possible to obtain the most appropriate light distribution characteristic which can be used for both the low beam and the upper beam. (Concerning this technique, for example, refer to JP-A-2003-257218.)
However, the movable shade described in JP-A-2003-257218 is pivotally provided so that it can be rotated round a rotary axis extending in the vehicle width direction. This rotary axis is located at a lower end portion of the movable shade, that is, this rotary axis is located at a position distant from the center of gravity of the movable shade. Accordingly, when the vehicle is vibrated, an upper end edge of the movable shade tends to be displaced. Therefore, a position of the cut-off line of the low beam light distribution pattern is likely to change.
In order to suppress the generation of such a displacement of the cut-off line of the low beam light distribution pattern, a return spring is built in an actuator for rotating the movable shade. By the action of this return spring, the movable shade can be urged toward the shading position at all times.
However, in order to effectively suppress the generation of this displacement, it is necessary to set the spring constant of the return spring at a considerably high value. Therefore, a drive force of the actuator, which is necessary for rotating the movable shade, is increased to a considerably high value. Accordingly, it is impossible to reduce the size and weight of the actuator, that is, it is impossible to decrease the manufacturing cost of the actuator.
In the projector type lighting device unit, since being restricted by its structure, an image of the light source composing the light distribution pattern is larger than an image of the parabola type lighting device unit. Therefore, in the case where a beam is changed over by the movable shade, especially in the case of irradiating the upper beam, a luminous intensity can not be raised sufficiently high.
Accordingly, an object of the present invention relates to solve the above problems.